Mica base insulating sheet and method for producing the same



Oct. 22, 1957 M. D. HEYMAN 2,810,425

MICA BASE INSJLATING SHEET AND METHQD FOR PRODUCING THE SAME Filed Feb. 10. 1954 INVENTOR.

M0555 7. HEYMAN ite States Pate MICA BASE INSULATING SHEET AND METHOD FR PRODUCING TIE SAME This invention relates to a sheet comprising an integrated mica base and a hard inorganic skeleton occupying the spaces or pores of said base. The invention also relates to a method for producing such a sheet. This application is a continuation-in-part of my pending application, Serial No. 286,700, filed May 8, 1952, now abancloned.

In the electronic field there are many places of application for disc-like elements that are strong yet light, hard yet non-abrasive, highly heat resistant, highly dielectric, and free of occluded gases or, at least, capable of being evacuated of any gases or air therein. example of use for such disc-like elements is in electronic tubes to support and space the various metal parts and fine wires therein. inasmuch as such tubes are under high vacuum and generate considerable heat, it will be evident that the mentioned properties are important for such discs, particularly the property of freely giving up any occluded gases when a vacuum is drawn in the tube. The hard and non-abrasive property is ,also important because it makes for good machineability and permits production by die cutting without abnormal wear of the dies and also provides an element that does not have undue wear or attrition on the fine metal parts in contact therewith. Those skilled in the art will readily understand the desirability of providing a sheet of the character indicated from which such disc'elements may be produced for the indicated and similar purposes. It is an object of the present invention to provide a mica base insulating sheet having the properties above mentioned.

The present sheet, in addition to the listed properties, is

form-retaining when immersed in liquid. The sheet also is cellular in its internal construction and, therefore, is hygroscopic. Consequently, the sheet may have use as a wick material in which the cells serve as capillaries. Another indicated use is as separator plates in storage batteries.

Another object of the invention is to provide a novel method for producing a sheet comprised of integrated mica in the spaces or pores of which a hard cellular skeleton of inorganic material is provided.

Another object of the invention is to provide such a method in which the inorganic material is conveyed to the pores of the integrated mica by an organic material and heat employed to evaporate or burn out all except the inorganic skeleton.

Reference is made to applicants Patent No. 2,405,576, dated August 13, 1946, in which integrated mica is disclosed.

The foregoing objects and other objects, features and advantages of the Vinvention will become more clearly evident from the following description of a preferred form of sheet and method for producing the same. It

is to be understood, however, that the description, which is based on the accompanying drawing, is given by way of example or illustration only.

ICC

In the drawing:

The figure is a greatly enlarged fragmentary sectional view of a mica base insulating sheet according to the present invention.

Integrated mica comprises extremely thin mica liakes having virgin or activated surfaces, said flakes being arranged in random fashion with their surfaces in contiguous relation enabling the natural cohesive forces resident in the akes to become effective to integrate the flakes into a sheet having spaces or pores among the flakes. The drawing shows such a sheet having fiakes 5, the same also showing that some of the flakes lie in more than one plane, and that spaces or pores 6 are formed among the liakes. The term integrated mica in this disclosure and in the appended claims is intended to define any sheet or mass of mica flakes or particles Vthat hold together in a handleable manner and in which said flakes or particles are integrated without a binder or other extraneous material.

The flakes comprising integrated mica, being substantially incompressible, the sheet itself cannot be compressed to such degree that the pores among the flakes are eliminated and the same are, therefore, retained even under application of pressure applied to the opposite faces. This incompressible property of integrated mica differs from the inherent compressibility of fibrous materials and is herein employed to provide a labyrinthine maze of spaces and passages capable of housing a hard cellular skeleton of inorganic-material in which the cells are retained.

According to the invention, the spaces or pores 6 are occupied by a skeleton 7 of inorganic material which embodies a multiplicity of cells, holes or pores 8. The problem has been to provide the inorganic skeleton to the substantial exclusion of all matter that would break down or gasify underY heat and/ or vacuum. During my work with this material, I have attempted to provide an inorganic skeleton by direct introduction into the pores 6 of silica in the form of glass particles of colloidal size but found that the resultant sheet was gritty yand highly abrasive, would unduly wear dies punching parts therefrom, and having wearing attrition on metal parts in contact therewith.

During my work with this material, I found that organo-inorganic material in a liquid state so as to be capable of impregnating integrated mica, whether or not vacuum is used, provides a satisfactory means for conveying inorganic material to pores 6 and that a suitable skeleton of inorganic material remains in said pores when the organic matter is removed. The liquid organo-inorganic material, which I have found will satisfactorily impregnate integrated mica'is commonly known as silicone resin which broadly comprises the metal siliconthe inorganic component-and a hydrocarbon-the organic component. These silicone resins have diverse formulae and, according to their formulae, different names or groups of names.

I have used as an impregnant siloxanes which are compounds of silicon, oxygen, carbon and hydrogen and containing in their molecules the structural unit RzSiO in which R is usually CH3 and may be 02H5, CsHrnror even more complex substituents. In commercial silicones R is usually CH3, i. e., they are methyl siloxanes.' These siloxanes are here given as merely an example of organosilicon compounds that are produced in the form of a resin capable of being held in suspension by various solvents, such as those of the group of aromatic solvents, e. g., benzene, toluene, xylene, naphtha, etc. A common silicone resin is methyl phenol polysiloxane.

Hereinafter l shall call an organo-inorganic material, such as above mentioned, and dissolved or in suspension in a solvent, a silicone resin, the same being intended as -a generic term for the organo-silicon impregnant employed in the following manner to -provide a skeleton 'of silica -in a porous sheet comprised of mica Hakes.

A fabricated sheet of mica is impregnated with silicone resin as by dipping the sheet in the resin until saturation is attained. For quicker saturation, -vacuum impregnation maybe employed. Time is then allowed for the solvent in the resin to evaporate. The drying time will depend Aon the proportion of solvent-in-thesolution. -In-thismanner, the silicon of the impregnant is brought into-the pores of the sheet since-silicone resin comprises both silicon and 'an organic material.

'The impregnated mica is then subjected to a temperature of approximately l000 LF. `Since mica may -dehydrate at temperatures higher than i000 F., -the same should not be exceeded vunless the possibilityof dehydration is not present. Itmay bestatedfthat thelheating -temperature may be raised to but-before the temperature of dehydrationl is reached. However, l000 F. has been found satisfactory.

In actual practice, the labove temperature is not provided immediately, but the impregnated sheet, after drying, is first baked at a temperature :between 400 F. to 500 F. for approximately one hour. At this stage, the impregnant still contains elements -of trapped moisture, air and gas, but a reaction has ltaken placethat-has-cured the silicone resin. Said action `thermo-sets the resininfthe fpores of the sheet.

After this baking period, the temperaturemaylbe raised tol000 F. and maintained for onehour tothree hoursor,l

as an alternative, -the temperature may be first raised `to 600 F., kept there for about one-half to :one hour-and then raised to 1000o F. forthe one 'to three hour rperiod. It is during this period of higher temperatureslthat the organic Imatter `or carbon in the resinand the .gases released thereby is burnt out to leave Ithe skeleton? with the multiplicity of cells or pores-8. Improved results may be obtained by increasing the bakingitime, as desired,.and as long as fortyeight hours. Thcheat above-mentioned 'may be .applied in the presenceof :oxygen to'improve elimination of the organic componentorrradicalA As a result of theforegoing method, the organic vcom-- that the same causes dehydration of the mica. Since such dehydration of natural mica releases moisture and generates steam, a reduction of the vacuum in the tubes results and the tubes rapidly deteriorate in their electronic function.

Dehydrated mica alone is much too soft for the abovedescribed use. By fortifying the sheet of mica flakes with successive impregnations of silica or comparable inorganic material, themass vof the Vsheet may be made to containas muchas twentypercent of silica. With such a large proportion of silica to fortify the mica, the sheet can be heated above the temperature at which the mica dehydrates to insure -substantially complete expulsion of all moisture. In practice, the heating range may be between 1400" and 1800" F. Also, the high-proportion of silica in the sheet renders the same stilic and strong. It has also been found that, by applying pressure to such a sheet while the same is being heated, the strength of the material is further increased, and said sheet has been found to have aftensile strength-up to twenty thousand pounds per square inch.

While this disclosure describes the :impregnation of a sheet from which disc-like parts :are to be punched or otherwise produced, the method may be carried out on mica parts-.already punched or formed and -not requiring further punching or machining operations. Therefore, the term sheet as used herein is intended to include such disc-like parts. v

While sI have described preferred products and methods that exemplify theinvention, the same are subject to modiiication Within the concepts of my invention. Therefore, I desire xto-reserve'tolmyself all modifications of product and method that may `fall within the spirit and scope of ythe-appended claims.

Having thus described my invention, what I claim and desire to obtain by Letters Patent, is:

-1. A sheet comprising a base of integrated mica iiakes arranged .in random fashion :and having therein a multiplicity of spaces, and a cellular skeleton of silica occupying 4said spaces.

ponent orcarbomradical in :the: silicone resin impregnant is burnt out and drivenoftthroughthecells-S; :a shellfor -skeleton of silica remains andlinessaid ,cellsythereispro- .vided a sheet thatisbard, quite stiifrand ofsuch strength as to have 'a tensile strength of 8500 p.fs.*i;;'and thcreis provideda sheet that:has the lproperties set-:fort-hiinV the preamble `to this speciiication.

il 4have also found that certain -silicatesfalthoughinot as efficient as silicone resins, may be;usedas=the,:impregnant. :These silicates are essentially;inorganics `as;distingui`shed from the-silicones which are 'organo-.inorganica theless, these-silicates, of which ethyl .silicate is an example, if 'suspended in a suit-able liquid carrier, suchas alcohol, maybe used to impregnate integrated -mica to carry silica-to thefpores of said micaf-and-will leave .a cellular silica 'skeleton after the carrier has -beenevaporated ont and the silica cured or thermo-set. l

From the foregoing,r it will be -`seenthat theinorganic material is brought into the pores of At-he mica basesheet bymeans ofeither-a liquid or solventrand organicmatter, or by-aliquid alone, and thatsuitable-.heat is fused to drive oit-isubstantially all matter and/gases fexeepbthe organic material. Therefore, the :present .method i employs a carrier r-for the v inorganic -material -that .thencomprises the skeleton Ywhich Voccupies the .fpores or; spaces of the mica Abase sheet. I,

The herein-'described sheet is hygroscopic becauseofthe cellular-:nature of'the silicaskeleton'l Y l'herefore,,said sheet is capable of beingagain impregnated `land burnt out, as described, Iand, if necessary, 4rse-'impregnated Vand burnt out a. third, fourth tand, even, a'ifthrtime. The resultant sheetisrfparticularly valuable Vfor,providingmica supports-or bridges in .electronic tubes-.that .operate.;atf ex tremely high ambient temperatures-temperatures so high 2. A sheet :comprising a'base of integrated mica iakes arranged in random fashion Iand vhaving therein a multiplicity of spaces, and a cellular skeleton comprising the silica component `of silicone fresin occupying said spaces.

3. A method fonproducing -a mica base insulating sheet that consists in impregnating a porous integrated sheet comprised of mica tiakes with a :solution consistingof .a silicone resin and a carrier material. of .volatile liquid, and,

Y While lretaining the porosity vof the sheet, :by ,application Neverof heat below the temperature at which the mica akes dehydrate and above the temperatureat which the silicone resin cures, driving od the carrier liquid to leaveV a cellular skeleton of silicain the poresV of the sheet.

4. A method for producing-a mica base insulating ,sheet that consists in impregnatng aV porous integrated sheet of vmica Vliakesin a solution containing silicone resinand,a solvent, evaporating the solvent to leave said silicone resin in the'pores of the sheet, applying heat tothesheet suicient to thermo-set the resin,l and, thereafter, applying rhigher heatthan the thermo-setting heat-to said sheet at a temperature below the tempera-ture at-,which ,the ^mica lakes dehydrate to `drive oigfrom said-thermo-,setgresin substantially all organic matter -contained ,therein to, thereby, leave-a cellular skeleton of silica; in said-pores.

Y5. A method according to v,claim 4: thelowheatpproximating 400 F. to 500 F.

i6. A method according to claim 4: the higherglieakap- .prox-imating :l000 fF. Y

7. A method according to claim 4: the low heat `approximating 400 F. to 500 F., and the higherheatap- .proximating l000 F.

Y8. VA method according to claim-4: the. step of impreg- ,mating cellsofthe skeleton with a silicone resin after the carrier liquid has been removed from said skeleton.

V9. A method according toclaim 3: the steps of imy amounts of silicone resin, and, nally, applying heat above the temperature at which mica dehydrates to expel a substantial proportion of the waters of crystallization from the sheet.

10. A method according to claim 9: the last mentioned temperature ranging between 1400 F. and 1800 F.

11. A method according to claim 9: the step of simultaneously applying pressure to the sheet while the ,lastmentioned heat is being applied.

12. In a method for producing a sheet comprised of integrated mica lakes and a cellular skeleton of silica among ,the akes, the steps of introducing silicone resin into the sheet in the company of a non-silica evaporative carrier, and, thereafter, applying heat su'cient to remove the carrier by evaporating the same and utilizing said heat to simultaneously eliminate the carbon radical of said resin to leave the mentioned skeleton of silica among the flakes.

13. In a method for producing a sheet comprised of integrated mica akes and a cellular skeleton of silica among the akes, the steps of introducing silicone resin into the sheet in the company of an evaporative organic material, and, thereafter, applying heat sucient to remove the organic material by evaporating the same and utilizing said heat to simultaneously eliminate the carbon radical of said resin to leave the mentioned skeleton of silica among the akes.

14. A method for producing a mica base insulating sheet that consists in impregnating a porous integrated sheet of mica flakes with ethyl silicate in solution in a volatile liquid, baking the impregnated sheet to thermoset the ethyl silicate while retaining the porosity of the sheet, and then subjecting the sheet to a temperature below the temperature at which the mica flakes dehydrate and above the temperature at which the organic component of said ethyl silicate is driven olf to, thereby leave a cellular skeleton of silica in the pores of the sheet.

References Cited in the le of this patent UNITED STATES PATENTS 1,578,812 Dawes Mar. 30, 1926 2,204,581 Denning June 18, 1940 2,352,974 Rochow July 4, 1944 2,405,576 Heyman Aug. 13, 1946 2,524,932 Schulman Oct. 10, 1950 2,546,474 Peyrot Mar. 27, 1951 

1. A SHEET COMPRISING A BASE OF INTEGRATED MICA FLAKES ARRANGED IN RANDOM FASHION AND HAVING THEREIN A MULTIPLICITY OF SPACES, AND A CELLULAR SKELETON OF SILICA OCCUPYING SAID SPACES. 